This invention relates to a casing structure formed of a plastic material and, more particularly, a structure of a casing having an improved shielding performance with respect to electromagnetic waves for accommodating electronic elements or equipment.
Recently, in accordance with the requirement for electronic equipment or instruments to be made compact, computerized or operated with high frequencies, a plastic material has been widely utilized for a material constituting a structure of a casing for accommodating the electronic equipment or instruments in view of design for the appearance and the reduction of the overall weight thereof.
However, in a case where the casing made of a plastic material is utilized for accommodating the electronic equipment, electromagnetic waves are generated from the accommodated equipment externally through the wall structure of the casing made of the plastic material and the thus generated electromagnetic waves adversely affect radio units, television sets or other electronic equipment or instruments disposed externally to the casing. Namely, in adverse cases, the radio unit may generate noises and disturbance may be caused to picture images of the television set due to the electromagnetic waves emitted from the equipment disposed in the casing. This equipment may eventually operate erroneously. Furthermore these adverse phenomena may adversely affect persons and, accordingly, some restrictions or regulations against the thus emitted electromagnetic waves have been made. Conversely, the electronic equipment accommodated in the plastic casing may be damaged by the electromagnetic waves from the external environment and in particular, the electromagnetic waves from the equipment or instruments using microcomputers will be significantly influenced.
In order to effectively eliminate the influence of the electromagnetic waves, a method has been provided in which a conductive coating is made on the inner wall surface of the plastic casing. In this method, in order to achieve an adequate electromagnetic wave shielding effect, it is necessary to keep the entire coated portion at a uniform electric potential and, desirably, to ground the electric conductive portions.
The casing is usually composed of two parts, i.e. a lid portion and a body portion, which are separable into two independent parts. Because of the separated structure, it is necessary to establish electrical conductivity between the conductive coat on the inner surface of the lid portion of the casing and the conductive coat on the inner surface of the body portion of the casing.
In order to achieve electrical conductivity therebetween, one conventional example provides a technique in which bosses are formed on the lid and body portions respectively and the bosses are electrically connected by means of vises, for example, to establish the electrically conductive conditions therebetween through the wire means. Another conventional example provides a structure of the casing in which a lid portion and a body portion are provided with lug portions which are to be connected when the lid and the body are mated and the surfaces of the lug portions are coated with a conductive material. A further conventional example provides a structure of the casing in which a lid portion is provided with a recess having a surface on which a conductive material is coated, and a body portion is provided with a protrusion having a surface on which a conductive material is coated. The protrusion and the recess are mated together to be electrically conductive when the lid is applied to the body.
With the first example described, however, it takes much time to assemble the casing and, hence, the workability thereof is not effective. With the second example, since the lug portions are formed on the outer surfaces of the lid and the body, it is necessary to take care with the arrangement of the casing. With the third example, it is necessary to coat the conductive material on the outer surface, as well as the inner surface, of the protrusion of the body of the casing and such coating working is troublesome. Moreover, the electrically conductive condition may not be established in cases where the protrusion of the body is not intimately contacted to the recess of the lid.